302 
FXUS63 KLSX 061107
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
607 AM CDT Mon Sep 6 2021

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 319 AM CDT Mon Sep 6 2021

High pressure remains firmly in control throughout the mid-
Mississippi Valley region early this morning, resulting in almost 
completely clear skies and light winds. Early morning satellite 
imagery indicates that fog has developed throughout many local river 
valleys, particularly in the Ozarks, although thus far there have 
been only a few instances of visibility reductions at local 
surface observation sites. Still, motorists can expected to 
encounter patchy fog early this morning, particularly in low lying
areas and near rivers. This fog should diminish quickly as mixing
commences after sunrise. Otherwise, sunny skies will continue 
today with temperatures climbing into the mid-80s, leading to 
excellent Labor Day weather. 

Overnight conditions will likely be similar to the previous night, 
although slightly increased southwesterly winds will keep 
temperatures slightly warmer, and may limit the coverage of fog. 
These southwest winds will continue to increase through the day, 
contributing to a slight warmup that will bring afternoon 
temperatures to near 90 degrees in most areas. As such, Tuesday is
expected to be the warmest day of the week for most areas. 

While dry conditions will persist through at least early afternoon, 
an approaching cold front may bring with it a slight chance of 
showers and perhaps a few thunderstorms late in the day. 
Throughout the day, a strengthening upper trough will dig into the
Great Lakes and drive this cold front through the mid-Mississippi
Valley between late afternoon and late evening. Ahead of the 
front, moisture content is expected to be somewhat limited due to 
the fact that low level trajectories will be cut off from the 
Gulf. However, moisture pooling ahead of the front may produce a 
narrow corridor of enhanced instability, although the degree to 
which this will occur remains somewhat uncertain. Some of the more
aggressive CAMs suggest that dewpoints may climb above 70 degrees
along this narrow corridor, leading to substantial instability 
projections, but given the previously mentioned limitations this 
is likely to be overstated. As such, these CAMs may also be overly
aggressive in initiating convection along the entire length of 
the front, particularly given the expected warm mid level 
temperatures (11-12 C at 700mb) and only limited forcing. Still, 
enough evidence now exists among CAMs and ensemble members to 
include a slight chance mention of thunderstorms across northeast 
and east-central Missouri, as well as west- central Illinois 
tomorrow evening. 

Should these storms materialize, they will likely struggle to fight 
through the convective inhibition for a time, and may never become 
particularly well organized. Just enough effective bulk shear 
(~35kt) will be in place that a more organized storm or two can't be 
completely ruled out, but given all of the uncertainties and 
limitations previously discussed, this will be unlikely. Shower 
chances will diminish overnight, and the cold front will continue to 
push south and out of the area.

BRC

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 319 AM CDT Mon Sep 6 2021

Following the cold front Tuesday night, northwest flow aloft and 
surface high pressure will reclaim the mid-Mississippi Valley 
region, reinforcing dry and mild conditions that will persist 
through Thursday as well. Temperatures both days will likely only 
reach the upper 70s to low 80s, with no precipitation expected and 
mostly clear skies as well. 

By Friday, a broad upper ridge will begin to expand eastward, and 
temperatures will begin a gradual warming trend that will continue 
into the weekend. Temperatures are expected to climb back to near 
average readings by Friday, to perhaps slightly above average over 
the weekend. Meanwhile, chances for precipitation will remain low 
through the weekend, thanks to the persistence of the upper ridge. 
While shortwaves will continue to traverse the northern periphery of 
the ridge, any precipitation they may generate is expected to remain 
well north of the area until at least early to mid week.

BRC

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 604 AM CDT Mon Sep 6 2021

Patchy valley fog has once again developed throughout the region,
but thus far has only temporarily impacted JEF and CPS with IFR
visibilities. Fog will quickly diminish after sunrise, and VFR
conditions are expected to persist through the remainder of the
12Z TAF period. Very patchy valley fog will be possible again
overnight tonight, but breezy southwest winds will likely restrict
fog to only the most sheltered low lying areas. 

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX